Fluorescent micro-particles embedded in a pigmented fluorescent coating for optical document security

ABSTRACT

A security feature ( 1 ) that is used with a substrate ( 2 ). The security feature includes a plurality of fluorescent micro-particles ( 1 A) that form a background component of the security feature and a plurality of fluorescent discrete particles ( 1 B) that form a foreground component of the security feature. The discrete particles are larger in size than the micro-particles and are optically distinguishable from the micro-particles under at least one illumination condition. In the preferred embodiment the security feature is applied as a coating to the substrate, and the security feature further includes a coating binder ( 1 C) in which the micro-particles and the discrete particles are contained. The coating binder can be made of, for example, at least one of an ink base, an adhesive, an epoxy, a varnish, a polymer solution, or a dry material having binding properties. The coating binder may be substantially transparent or substantially opaque. The discrete particles are optically distinguishable from the micro-particles under long wavelength ultraviolet light and/or under short wavelength ultraviolet light. Under no or low magnification conditions the security feature exhibits a generally uniform color to the naked eye, while in one embodiment under higher magnification conditions the discrete particles exhibit a first color while the micro-particles of the background component exhibit a second color, while in another embodiment the discrete particles exhibit a first color while the micro-particles of the background component exhibit a lack of color, while in a further embodiment the micro-particles of the background component exhibit a first color while the discrete particles exhibit a lack of color. The security feature exhibits a fluorescent signature having attributes given by at least one of particle count per unit area, specific particle size range which may or may not differ by fluorescent color, relative intensities of measured spectral emission peaks due to foreground component particle fluorescence versus background component micro-particle fluorescence and specific emission wavelength values of measured spectral emission peaks due to foreground component particle fluorescence versus background component micro-particle fluorescence.

TECHNICAL FIELD

[0001] These teachings relate generally to the use of fluorescentpigments and materials and, in particular, to the use of fluorescentpigments in applications where it is desired to provide authenticationand/or security function for documents, banknotes, financial instrumentsand the like.

BACKGROUND

[0002] Coatings containing fluorescent pigments are currently used tocreate threads, fibers, and other security structures that areincorporated into the manufacture of banknote, financial, and legaldocument papers. These structures may be though of as security featuresthat serve as optical authentication devices, as well as to preventdocument counterfeiting. Such coatings may also be applied to productpackaging to identify authentic products and deter productcounterfeiting. The fluorescence of such coatings is typically notapparent when illuminated with ordinary ambient light sources, butbecomes visible when excited by an ultraviolet light source.

[0003] Reference can be had to the following U.S. Pat. Nos.: 5,903,340,“Optically-Based Methods and Apparatus for Performing DocumentAuthentication”, by N. M. Lawandy and T. J. Driscoll; and 4,863,783,“Security Paper”, by N. A. Milton. Reference can also be had to EuropeanPatent Application 0 219 743 A1, “Security Paper Containing VesiculatedBeads, by R. H. Hamilton; and to French Patent No.: 2 478 695,“Antifalsification Paper with Luminescent Particles, its Method ofProduction and Method of Producing Said Particles” (Aussedat Rey).

[0004] For example, Milton describes a paper that contains 30-500 micronsize granules of 3-5 micron size pigment particles that are chemicallybound together by a cross-link binder. In order to provide contrastbetween the pigment and the background during inspection the granulesare essentially free of finer particles.

[0005] In French Patent No.: 2 478 695 a paper is provided that includesluminescent particles that are a 30-50 micron size conglomerate of 3-5micron size pigment particles. The particles may emit differentwavelengths when exposed to UV radiation, and combinations of colors aresaid to enable the papers to be personalized. In addition to the use ofthe fluorescent particles, the paper may also contain colored fibers andother conventional means of identification.

[0006] A problem that has not been adequately addressed by the prior artlies in complicating the task of the counterfeiter or forger byincreasing the complexity of the optically-based security feature.

SUMMARY OF THE PREFERRED EMBODIMENTS

[0007] The foregoing and other problems are overcome, and otheradvantages are realized, in accordance with the presently preferredembodiments of these teachings.

[0008] In accordance with the teachings of this invention there areprovided non-apparent fluorescent microscopic particles that are presentwithin a coating for the purposes of increasing a maximum number ofunique fluorescent emission combinations, also referred to herein asunique fluorescent signatures, for secreting these signatures from readydiscovery, and for increasing the difficulty of creating counterfeitdocuments and product packaging.

[0009] These teachings are an advancement over the prior art since theiruse complicates the task of the counterfeiter or forger by increasingthe complexity of an optically-based security feature. These teachingsprovide a security paper and substrate with an efficient multi-levelencoding mechanism using particles containing pigment that emit lightwith different wavelengths and/or that emit or do not emit lightdepending on the characteristics of the stimulus. The use of lightemitting particles within at least two different size regimes increasesthe complexity of the security feature and furthermore increases theamount of information that can be encoded by the light emittingparticles.

[0010] Disclosed herein is a security feature that is used with asubstrate. The security feature includes a plurality of fluorescentmicro-particles that form a background component of the security featureand a plurality of fluorescent particles that form a foregroundcomponent of the security feature. The particles are larger in size thanthe micro-particles and are optically distinguishable from themicro-particles under at least one illumination condition. For example,the particles may have a size in the range of about 10 microns to about20 microns in diameter and the micro-particles may have a size in therange of about 0.2 microns to about 2 microns in diameter. In general,and further by example, an average size of the particles is greater by afactor of at least five than an average size of the micro-particles, orthe average size of the particles is greater by a factor of at least oneorder of magnitude than the average size of the micro-particles.

[0011] In the preferred embodiment the security feature is applied as acoating to the substrate, and the security feature further includes acoating binder in which the micro-particles and the particles arecontained. The coating binder can be made of, for example, at least oneof an ink base, an adhesive, an epoxy, a varnish, a polymer solution, ora dry material having binding properties.

[0012] The coating binder may be substantially transparent orsubstantially opaque. The particles are optically distinguishable fromthe micro-particles under long wavelength ultraviolet light (320-380 nmrange, peaking at ˜365 nm) and/or under short wavelength ultravioletlight (180-280 nm range, peaking at ˜254 nm). Under no or lowmagnification conditions the security feature exhibits a generallyuniform color to the naked eye, while in one embodiment under highermagnification conditions the discrete particles exhibit a first colorwhile the micro-particles of the background component exhibit a secondcolor, while in another embodiment the discrete particles exhibit afirst color while the micro-particles of the background componentexhibit a lack of color, while in a further embodiment themicro-particles of the background component exhibit a first color whilethe discrete particles exhibit a lack of color. In the presentlypreferred embodiment the security feature exhibits one of many possibleunique fluorescent signatures having attributes given by at least one ofparticle count per unit area, specific particle size range which may ormay not differ by fluorescent color, relative intensities of measuredspectral emission peaks due to foreground component particlefluorescence versus background component micro-particle fluorescence andspecific emission wavelength values of measured spectral emission peaksdue to foreground component particle fluorescence versus backgroundcomponent micro-particle fluorescence. In the preferred, but notlimiting embodiment, the discrete particles are invisible or nearlyinvisible to the naked eye.

[0013] Also described is an automatic reader system for resolving andidentifying the various spectral and spatial properties of the securityfeature.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The foregoing and other aspects of these teachings are made moreevident in the following Detailed Description of the PreferredEmbodiments, when read in conjunction with the attached Drawing Figures,wherein:

[0015]FIG. 1 is graph showing an exemplary emission spectrum for ayellow-red system film;

[0016]FIG. 2A shows the film system of FIG. 1 as seen by the naked eye(magnification 1×), while FIG. 2B shows a microscopic view of the filmsystem (magnification 100×);

[0017]FIG. 3 is a system-level block diagram of a reading system forstimulating the particles contained within a substrate and for readingthe resulting emission(s) of light.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] This invention provides a novel approach for enhancing thesecurity of documents, packaging and other substrates by includingnon-apparent fluorescent microscopic particles into a coating and/ordirectly into the substrate, for the purposes of increasing the numberof unique fluorescent signatures possible, for secreting thesesignatures from ready discovery, and for increasing the difficultyfactor in creating counterfeit documents and product packaging. Whiledescribed herein in the general context of paper-based documents, suchas legal documents (e.g., deeds and contracts) and financial documents(e.g., currency, stock certificates, bonds and letters of credit), theseteachings may be employed with any of a number of different types ofdocuments. These teachings may also be employed with other thanpaper-based substrate materials, where the term “substrate” should bebroadly construed to include any type of substrate material that issuitable for being printed on, including ceramic, polymeric, plastic andplastic-containing substrate materials.

[0019] There are a number of possible embodiments of these teachings.These embodiments include, but are not limited to, the followingembodiments shown by way of Examples A through I.

EXAMPLE A

[0020] Particles excited by Long Wavelength Ultraviolet (Long UV) lightemit a fluorescent color X (for example, yellow, where the particlescontain a 2-5 percent mixture of a fluorescent pigment of thebenzothiazolyl family embedded in a clear, hard plastic matrix, or ofsimilar construct using fluorescent pigments of other colors) and have aspecific size range (for example, about 10 microns to about 20 micronsin diameter). These particles are also referred to herein as discreteparticles. A coating binder may be comprised of, for example, an inkbase, an adhesive, an epoxy, a varnish, a polymer solution, or a drypowder material having binding properties. The coating binder contains aLong UV fluorescent pigment having an emission color Z (for example, ared fluorescent pigment such as a Europium chelate), with the pigmentbeing milled into the coating to a much smaller particle size range (forexample, 0.5 micron to 2.0 microns, also referred to herein asmicro-particles), or being fully dissolved into the coating. Thisbackground field pigment loading in the coating may range from 2-20% byweight, with a preferred loading at about 10% (wet basis), so to resulta dry basis loading of about 10-30%. For example, the average size ofthe discrete particles can be greater by a factor of at least five thanthe average size of the micro-particles. Further by example, the averagesize of the discrete particles can be greater by a factor of at leastone order of magnitude than the average size of the micro-particles. Thepreferred loading of the discrete particles in a given film may varybetween about 0.2-10% (wet basis) so to result in a dry basis loading ofabout 0.2-30%. Such wide variation of this attribute contributes to thelarge number of possible unique signatures within any single colorcombination or other single embodiment of the security feature, wheresuch attribute is measurable as discrete particle count per unit area ofcoated or co-manufactured substrate.

[0021] The 10-20 micron sized discrete particles are combined and mixedwith the coating binder and its micro-particles, or the discreteparticles and the micro-particles could be added together into thecoating binder, or they can be added in either order into the coatingbinder, and then mixed. In any case, the resulting liquid or solid phasecoating material containing the discrete particles and themicro-particles is applied as a thin film to a substrate of interest.The coating material can be applied by painting on, rolling on, sprayingon, through commercial printing techniques, or by any suitableapplication process, such as by writing the coating material onto thesubstrate using a pen or a marker (e.g., a felt-tip type marker) thatcontains the coating material.

[0022] The net visual effect to the naked eye, or under lowmagnification, is a single emission color Y (orange), which is thevisually averaged combination of the yellow and red fluorescentemissions. However, inspection of the coating film under highermagnification reveals the presence of the discrete particles having thefluorescent color X (yellow in this example) disposed within a fieldcomprised of micro-particles having fluorescent color Z (red in thisexample). If the coating binder is itself transparent, a thin film ofthis coating is translucent to transparent (substantially transparent)when viewed under ordinary ambient light by the naked eye.

[0023] An emission spectrum (as seen by the naked eye) of such ayellow-red system film is shown in FIG. 1. A drawing representing theappearance of the same film (as viewed by the naked eye vs. through amicroscope) is shown in FIGS. 2A and 2B. FIG. 2B shows themicro-particles 1A, discrete particles 1B, and the coating binder 1C.

EXAMPLE B

[0024] Discrete particles excited by either Long or Short UV light emita fluorescent color X (for example, blue) and have a specific size range(for example, 10 microns to 20 microns). The coating binder can be anyone described above in Example A. The coating contains a Long UV/ShortUV color-shifting background pigment blend having Long UV emission colorX (for example, a blue emitting pigment of the stilbene family) andShort UV emission color Z (for example, an inorganic red emittingpigment such as Uveda™ YO obtained from United Mineral Corporation).Such a blend of background pigments may contain a ratio of between 1:5and 1:10 Long UV: Short UV pigments. The blend is milled into thecoating to a final size range (micro-particles of, for example, 0.5micron to 2.0 microns) smaller than the discrete particles, then thediscrete particles and micro-particles are added and thoroughly mixedinto the coating (solution or dry powder). When a thin film of thiscoating is excited under Long UV light and viewed either by the nakedeye or under high magnification, the net appearance is a flat fieldhaving emission color X (blue). Outlines of the discrete particles arevisible under magnification. When the film is excited under Short UVlight and viewed by the naked eye the net appearance is a flat fieldhaving emission color Y (violet). When viewed under high magnification,however, the presence of discrete particles having fluorescent color X(blue) standing in a field of fluorescent color Z (red) is revealed. Ifthe coating binder used is transparent, this coating is translucent totransparent when viewed under ordinary ambient light by the naked eye.

EXAMPLE C

[0025] Discrete particles excited by Long UV light emit fluorescentcolor X (for example, blue), and when excited by Short UV light emit adifferent fluorescent color Y (for example, red) and have a specificsize range (for example, 10 microns to 20 microns). The coating bindercan be any one described in above in Example A. The coating contains aLong UV background pigment having emission color Z (for example,yellow), but having very little or no emission when excited by a ShortUV light source. Again, the background pigment is milled into thecoating and has a smaller size range (micro-particles of, for example,0.5 micron to 2.0 microns) than the particles. The discrete particlesand micro-particles are added and thoroughly mixed into the coating(solution or dry powder). When a thin film of this coating is excitedunder Long UV light and viewed by the naked eye, the net appearance is aflat field having emission color W (green, in this case). When viewedunder high magnification the presence of the discrete particles havingfluorescent color X (blue) standing in a field of fluorescent color Z(yellow) is revealed, similar to Example A above. However, when thisfilm is excited under Short UV light and viewed by the naked eye the netappearance is a disperse field having emission color Y (red). Whenviewed under high magnification the presence of discrete particleshaving fluorescent color Y (red) standing in a non-fluorescingbackground field is revealed. If the coating binder used is transparent,this coating is translucent to transparent when viewed under ordinaryambient light by the naked eye.

EXAMPLE D

[0026] In this example the discrete particles emit Long UV color X (forexample, yellow), but do not fluoresce under Short UV excitation. Thecoating binder can be any one described in above in Example A. Thecoating contains a Long UV/Short UV color-shifting background pigmenthaving Long UV emission color Y (for example, blue) and Short UVemission color Z (for example, red). Again, the background pigment ismilled into the coating and has a smaller size range (micro-particlesof, for example, 0.5 micron to 2.0 microns) than the particles. Theparticles and micro-particles are added and thoroughly mixed into thecoating (solution or dry powder). When a thin film of this coating isexcited under Long UV light and viewed by the naked eye, the netappearance is a flat field having emission color W (green, in thiscase). When viewed under high magnification the presence of discreteparticles having fluorescent color X (yellow) standing in a field offluorescent color Y (blue) is revealed, similar to Example A above.However, when this film is excited under Short UV light and viewed bythe naked eye the net appearance is a disperse field having emissioncolor Z (red). When viewed under high magnification the presence ofdiscrete particles having no fluorescence (dark spots) standing in abackground field of fluorescent color Z (red) is revealed. If thecoating binder used is transparent, this coating is translucent totransparent when viewed under ordinary ambient light by the naked eye.

EXAMPLE E

[0027] In this example non-fluorescing discrete particles were selectedthat may be opaque, translucent, or transparent, and have a specificsize range (for example, 10 microns to 20 microns). The coating bindercan be any one described in above in Example A, and the backgroundpigment may be as described above in either Example A or Example B. Whena film of this coating is excited under Long UV light and viewed by thenaked eye the net appearance is a flat field having Long UV emissioncolor X (for example, green). When viewed under high magnification thepresence of discrete particles having no fluorescence (dark spots)standing in a background field of fluorescent color X (green) isrevealed. If the background pigment is as described in Example B, andthe film is excited under Short UV light, when viewed by the naked eyethe net appearance is a flat field having Short UV emission color Y (forexample, orange). When viewed under high magnification the presence ofdiscrete particles having no fluorescence (dark spots) standing in abackground field of fluorescent color Y (orange) is revealed. If thecoating binder used is transparent, this coating is translucent totransparent when viewed under ordinary ambient light by the naked eye.

EXAMPLE F

[0028] Fluorescent discrete particles were selected having emissionproperties as described in any of Examples A, B, C or D, and have aspecific size range (for example, <10 microns). A transparent coatingbinder is used, such as one of those described in Example A, but thecoating contains no background pigment. When a film of this coating isexcited under appropriate (Long and/or Short, per particle properties)UV light and is viewed by the naked eye the net appearance is a dispersefield having emission color X (for example, green). When viewed underhigh magnification the presence of discrete particles having fluorescentcolor X (green) standing in transparent background field is revealed.When viewed under ordinary ambient light by the naked eye, this coatingis transparent (invisible).

EXAMPLE G

[0029] Fluorescent discrete particles were selected having emissionproperties as described in any of Examples A, B, C, or D, and have aspecific size range (for example, <10 microns). A transparent coatingbinder is used, such as described in Example A, and the coating containsa non-fluorescent, substantially opaque background pigment (for example,black). When a film of this coating is excited under appropriate (Longand/or Short, per particle properties) UV light and is viewed by thenaked eye the net appearance is a disperse field having emission color X(for example, green). When viewed under high magnification the presenceof discrete particles having fluorescent color X (green) standing in anopaque (black) background field is revealed. When viewed under ordinaryambient light by the naked eye, this coating is opaque (black).

EXAMPLE H

[0030] A mixture of two or more different fluorescent colored discreteparticles is used having emission properties as described in any ofExamples A, B, C, D or E, and having a predetermined size range (whichmay or may not differ according to particle fluorescent color). Thecoating binder can be as described in Example A, and the coating maycontain any fluorescent or other background field pigment havingproperties as described in Examples A, B, C, D, E, F, or G. When a thinfilm of this coating is excited under appropriate (Long and/or Short,per particles and background micro-particle properties) UV light andviewed by the naked eye, the net appearance is a flat field having asingle color (being the sum of emission wavelengths and intensities asintegrated by the human eye). When viewed under high magnification, thepresence of discrete particles having various fluorescent colors isrevealed. The background pigment field color may or may not be of thesame as any one of the particle colors.

EXAMPLE I

[0031] In accordance with a further aspect of these teachings a coatingcontaining a binder as described in Example A, and containing anyfluorescent or other background field pigment having properties asdescribed in Examples A, B, C, D, E, F, or G, is first applied as a basefilm and allowed to dry or cure. A second coating, as described inExample F above, except that this coating may instead contain discreteparticles as described in Example H, is applied as a thin film to thetop surface of the base film. In this case, the discrete particles arenot embedded in the pigmented base coating, but are surface mountedabove the base coat. To the naked eye, when illuminated underappropriate UV light source from directly above, such binary coatingsystem film appears as a flat field having a single color (being the sumof emission wavelengths and intensities as integrated by the human eye),but assumes a speckled or disperse appearance as the incident angle ofUV illumination is changed. When viewed under high magnification, thelayer of discrete particles in the top coat will be at a shorter focallength than the background pigment field color in the base coat.

[0032] The creation of a specific and unique fluorescence signature inany of the above Examples A-I is accomplished through a balance ofpigment concentrations contained in the particles of the foregroundcomponent of the security feature coating and the micro-particles of thebackground component of the security feature coating, the overallconcentrations of particles and of the background micro-particles, theexcitation range(s) selected, and the fluorescent color(s) selected, allwithin an appropriate application thickness of the coating film.

[0033] Attributes of such fluorescent signature may include particlecount per unit area, specific particle size range (which may or may notdiffer by fluorescent color for the case of Example H), net spectralcharacteristic relative intensities of measured UV emission peaks due toparticle fluorescence vs. background pigment fluorescence, specificemission wavelength values due to particle fluorescence vs. backgroundpigment fluorescence, and other characteristics such as particledensity(s) and background field pigment concentration. For a givencoating formulation, the values of certain of these attributes will varyas a function of applied film thickness.

[0034] Referring to FIG. 3, a security feature 1 as described above inthe Examples A-I is provided on a substrate 2 and may be coupled with anautomated reading device 10 to authenticate the presence of the securityfeature 1 and/or to verify its specific fluorescent signature. Thereading device 10 can include an image collection element 12, which mayinclude a digital or video camera image capture device 14, magnificationoptics 16, a physical standoff 16A to readily establish a correct focaldistance between the optics 16 and the target coating film of thesecurity feature 1 on the substrate 2, and appropriate UV targetillumination source(s) 18 (Long UV) and 20 (Short UV). The readingdevice 10 also includes a measurement element 22 that can include aspectrometer 24 and image and spectral data processing elements 26, suchas a real-time or captured image display 28, a data link 30 to acomputer 32 having a memory 34, and image processing software 36 forevaluating the target image and spectra, and for comparing their valueswith signature-specific allowed value ranges. The memory 34 can store alookup table or other type of data structure 34A wherein variousauthentic combinations of background and foreground colors, particlesizes, particle densities and the like are stored.

[0035] As an example, a particular document may be provided with anoptical signature given by a thin film coating security featurecontaining background micro-particles that fluoresce with a yellow colorwhen excited by Long UV, and that do not have any appreciablefluorescent emission when excited by Short UV, and with discreteparticles of size about 20 microns that fluoresce with a red color whenexcited by Long UV and with a yellow color when excited by Short UV,where the particles have a density of about 10/mm².

[0036] In this case the reading device 10 may be automatically operatedso as to first illuminate the coating with the Long UV light source 18,and to detect the discrete yellow and red emissions, then to illuminatethe coating with the Short UV light source 20, and to detect thediscrete yellow emission (but of smaller magnitude than the magnitude ofthe yellow emission due to the background micro-particles when exited bythe Long UV). The output of the image capture device 14 may also beprocessed to confirm that the red and yellow emissions originate fromparticles having a size of about 20 microns and a density of about10/mm². If all of these criteria are satisfied then the substrate maybedeclared to be genuine or authentic, as the security feature isvalidated against data stored in the data structure 34A. A further test,or an initial test, may be one performed by a human operator whoverifies that the expected color(s) are present under different Long andShort UV illumination conditions. It can be appreciated that theseteachings provide a multi-dimensional security feature (e.g., excitationsource wavelength(s), emission wavelength(s), particle size(s), particledensity(s), and emission amplitude(s)).

[0037] While described in the context of the use of the two UV sources(Long and Short) 18 and 20, a single tuneable output UV source could beused. Also, these teachings can be employed with a single UV source,e.g., either Long or Short, or with more than two UV sources, forexample one that covers at least the 280 nm to 320 mn between the ShortUV and the Long UV ranges. Also, these teachings are not restricted foruse with only the described pigments, particle and micro-particle sizes,ranges of particle and micro-particle sizes and/or types of coatingbinders. Furthermore, while described in the context of a coating thatis applied to form a thin film on the substrate 2, it is also within thescope of these teachings to incorporate at least the micro-particles andthe particles into the substrate, such as by adding them during thesubstrate manufacturing process.

What is claimed is:
 1. A security feature that is used with a substrate,comprising: a plurality of fluorescent micro-particles that form abackground component of said security feature; and a plurality offluorescent discrete particles that form a foreground component of saidsecurity feature, said discrete particles being larger in size than saidmicro-particles and being optically distinguishable from saidmicro-particles under at least one illumination condition.
 2. A securityfeature as in claim 1, wherein said security feature is applied as acoating to said substrate, and further comprising a coating binder inwhich said micro-particles and said discrete particles are contained. 3.A security feature as in claim 2, wherein said coating binder iscomprised of at least one of an ink base, an adhesive, an epoxy, avarnish, a polymer solution, or a dry material having bindingproperties.
 4. A security feature as in claim 1, wherein said discreteparticles are optically distinguishable from said micro-particles underlong wavelength ultraviolet light.
 5. A security feature as in claim 1,wherein said discrete particles are optically distinguishable from saidmicro-particles under short wavelength ultraviolet light.
 6. A securityfeature as in claim 1, wherein under no or low magnification conditionsthe security feature exhibits a generally uniform color to the nakedeye, while under higher magnification conditions the discrete particlesexhibit a first color while the micro-particles of the backgroundcomponent exhibit a second color.
 7. A security feature as in claim 1,wherein under no or low magnification conditions the security featureexhibits a generally uniform color to the naked eye, while under highermagnification conditions the discrete particles exhibit a first colorwhile the micro-particles of the background component exhibit a lack ofcolor.
 8. A security feature as in claim 1, wherein under no or lowmagnification conditions the security feature exhibits a generallyuniform color to the naked eye, while under higher magnificationconditions the micro-particles of the background component exhibit afirst color while the discrete particles exhibit a lack of color.
 9. Asecurity feature as in claim 1, wherein said security feature exhibits afluorescent signature having attributes given by at least one ofparticle count per unit area, specific particle size range which may ormay not differ by fluorescent color, relative intensities of measuredspectral emission peaks due to foreground component particlefluorescence versus background component micro-particle fluorescence andspecific emission wavelength values of measured spectral emission peaksdue to foreground component particle fluorescence versus backgroundcomponent micro-particle fluorescence.
 10. A security feature as inclaim 1, wherein said discrete particles have a size in the range ofabout 10 microns to about 20 microns in diameter.
 11. A security featureas in claim 1, wherein said micro-particles have a size in the range ofabout 0.2 microns to about 2 microns in diameter.
 12. A security featureas in claim 1, wherein an average size of said discrete particles isgreater by a factor of at least five than an average size of saidmicro-particles.
 13. A security feature as in claim 1, wherein anaverage size of said discrete particles is greater by a factor of atleast one order of magnitude than an average size of saidmicro-particles.
 14. A security feature as in claim 2, wherein saidcoating binder is substantially transparent.
 15. A security feature asin claim 2, wherein said coating binder is substantially opaque.
 16. Asecurity feature as in claim 1, wherein said discrete particles areoptically distinguishable in a first way from said micro-particles whenviewed under short wavelength ultraviolet light, and are opticallydistinguishable in a second way from said micro-particles when viewedunder long wavelength ultraviolet light.
 17. A reader system comprisinga plurality of light sources and a magnification optics coupled to animaging system, said reader system comprising a controller forilluminating a security feature using said plurality of light sources,said security feature comprising a plurality of fluorescentmicro-particles that form a background component of said securityfeature and a plurality of fluorescent discrete particles that form aforeground component of said security feature, said discrete particlesbeing larger in size than said micro-particles and being opticallydistinguishable from said micro-particles when illuminated by at leastone of said plurality of light sources, said controller cooperating withsaid imaging system for resolving a fluorescent signature of saidsecurity feature, said fluorescent signature having attributes given byat least one of particle count per unit area, specific particle sizerange which may or may not differ by fluorescent color, relativeintensities of measured spectral emission peaks due to foregroundcomponent particle fluorescence versus background componentmicro-particle fluorescence and specific emission wavelength values ofmeasured spectral emission peaks due to foreground component particlefluorescence versus background component micro-particle fluorescence.18. A reader system as in claim 17, wherein said security feature isapplied as a coating to a substrate using a coating binder in which saidmicro-particles and said discrete particles are contained.
 19. A methodfor reading a security feature, comprising steps of: operating a readersystem having a plurality of light sources and magnification opticscoupled to an imaging system for illuminating the security feature usingsaid plurality of light sources, said security feature comprising aplurality of fluorescent micro-particles that form a backgroundcomponent of said security feature and a plurality of fluorescentdiscrete particles that form a foreground component of said securityfeature, said discrete particles being larger in size than saidmicro-particles and being optically distinguishable from saidmicro-particles when illuminated by at least one of said plurality oflight sources; and responsive to the illuminating, resolving afluorescent signature of said security feature, said fluorescentsignature having attributes given by at least one of particle count perunit area, specific particle size range which may or may not differ byfluorescent color, relative intensities of measured spectral emissionpeaks due to foreground component particle fluorescence versusbackground component micro-particle fluorescence and specific emissionwavelength values of measured spectral emission peaks due to foregroundcomponent particle fluorescence versus background componentmicro-particle fluorescence.
 20. A method as in claim 19, and comprisinga preliminary step of applying said security feature as a coating to asubstrate using a coating binder in which said micro-particles and saiddiscrete particles are contained.